The proposed research is designed to examine the effects of aging on the functional dynamics of the pituitary-adrenal system, using the laboratory rat as a model. Specifically, we propose to investigate age-related changes in the adrenocortical response to psychological variables that have been shown to produce either excitatory or inhibitory effects on pituitary-adrenal activity in presenescent animals, and the capacity of the aged rat to re-entrain the daily pattern of corticosteroid rhythmicity in response to environmental cues. Systematic investigation of possible sex differences in the pituitary-adrenal responses of aged animals will be included in each proposed experiment. Age-related changes in behavior will also be investigated and related to the observed patterns of pituitary-adrenal activity. The indicant of pituitary-adrenal response in each of the proposed experiments will be plasma levels of corticosterone.